1. Field of the Invention
The present invention relates to a combined scanning electron and scanning tunnelling microscope apparatus, and to a method of scanning using such an apparatus. In such an apparatus, a scanning tunnelling microscope has its probe locatable in the scanning area of a scanning electron microscope, to enable the scanning area of the probe of the scanning tunnelling microscope to be determined.
2. Summary of the Prior Art
A scanning tunnelling microscope (hereinafter referred to as a "STM"), has a probe, normally of thin metallic material. The probe is normally positioned so that its longitudinal axis is substantially perpendicular to the sample, and the probe is movable in that axial direction. When the probe is brought very close to the surface of the sample (e.g. about 1 nm), and a predetermined voltage is applied between the probe and the sample, a tunnel current may be generated. By controlling the separation between the probe and the surface of the sample, so that the separation is constant, a constant tunnel current will be achieved. Therefore, if the probe is caused to scan over the surface of the sample, either one-dimensionally or two-dimensionally, and the location of the probe is controlled to maintain the tunnel current constant, the movement of the probe in its axial direction represents the surface irregularity of the sample.
From this movement, an image of the surface irregularity of the sample can be achieved. An STM has an extremely high spatial resolution, and this is sufficiently high to distinguish atomic structures, so that the image will give information concerning the atomic structure of surface of the sample. The basic principles of such an STM are described in e.g. "Scanning Electron Microscopy"/1983/III pages 1070-1082.
Although an STM has a high spatial resolution, its scanning range is extremely narrow, with a maximum of about 10 um. Thus, the probe of the STM must be located precisely on a target region of the sample, and it is not practically possible to use the STM itself to locate the position of that target. Therefore, it is necessary to use another microscope of a different type to pre-locate the probe relative to the target of the sample before the STM can carry out its scanning operation.
If an STM is combined with an optical microscope, the gap between the objective lense of the optical microscope and the surface of the sample is too small to allow observation of both the sample and the probe simultaneously. Attempts have been made, in e.g. "Industrial Applications of Charge Particle Beams" from The Japan Society for the Promotion of Science, 132 Committee, 109 Study Report, pages 41-46, to make use of an optical microscope in conjunction with an STM, but the optical and geometrical arrangements needed are complex.
Therefore, it has been proposed to combine an STM with a scanning electron microscope (hereinafter referred to as an "SEM"). With an SEM, it is possible to obtain an image of both a part of the sample and the probe of the STM. Examples of such a combination, producing a combined scanning electron and scanning tunnelling microscope apparatus are shown in e.g. JP-A-63-298951 and the article entitled "Scanning Tunnelling Microscope Combined with Scanning Electron Microscope" by T Ichinokawa et al in Ultramicroscopy 23(1987) pages 115 to 118.
FIG. 1 of the accompanying drawings shows a simplified apparatus in which a SEM and STM are combined. In FIG. 1, a sample 100 is mounted adjacent a probe 101 of a STM, the sample 100 being mounted on a support table 102. The longitudinal axis of the probe 101 is substantially perpendicular to the surface of the sample 100, and the probe is movable along that axis. A SEM 103 is mounted adjacent the sample 100 and the probe 101, and generates an electron beam 104 directed towards the sample 100. It can be seen that the table 102 is inclined relative to the beam 104, and the table 102 is designed to permit relative movement of the sample 100 and probe 101 perpendicular to the longitudinal axis of the probe 101. Furthermore, the probe 101 and table 102 are mounted on a support 105 which itself may be movable relative to the SEM 103. When the beam 104 is caused to scan the sample 100, the probe 101 may be moved into the scan area of the beam 104, so that a part of the sample 100 and the probe 101 can be observed simultaneously on a display screen of the SEM.
Of course, there still remains the problem that the probe 101 must be located at the target site on the sample 100 before the STM can operate. In the article entitled "SEM/STM Combination for STM Tip Guidance" by M Anders et al in Ultramicroscopy 24 (1988) pages 123-128, it was suggested that use should be made of a pattern of markers on the sample, which markers on the sample had known positions relative to the target. Thus, if the operator saw one of those markers in the display of the SEM, he would be able to find the location of the target itself. Indeed, in that article it was proposed that the target itself be used as the marker, if it was sufficiently recognisable.
This arrangement enable the target to be located, but there is then a further problem. FIG. 2 of the accompanying drawings shows schematically the display image of the SEM, displaying an image 100a of part of the sample 100 in FIG. 1, and an image 101a of the tip of the probe 101. The display has no parallax between the image 100a of the part of the sample and the image 101a of the tip of the probe 101. Therefore the display does not clearly indicate the relative positions of the sample and probe. When the sample 100 is moved relative to the probe 101, it is important that the tip 101a of the probe is spaced by a sufficient distance from the sample so that surface irregularities in the sample will not cause contact between the sample and the tip of the probe. However, when the STM operates, the tip must be around 1 nm from the sample surface. Therefore, it is not possible from the display image for the operator to be able to determine, during movement of the sample 100 relative to the probe 101, whether the probe 101 will be accurately aligned with the target point on the sample when the probe is moved axially to its scanning distance.
In an article entitled "Surface Investigations with a Combined Scanning Electron-Scanning Tunnel Microscope" by Fuchs et al, in Scanning Vol. 12 pages 126 to 132, it was proposed to make use of a shadow arrangement. In the operation of an SEM, the image is generated from electrons from the sample, and these electrons may be back-scattered electrons or secondary electrons. The article by H Fuchs et al proposed to suppress the secondary electrons, and make use of the back scattered electrons, as it was found that these would generate a shadow of the probe tip on the sample. Therefore, the display of the SEM showed the sample, the tip of the probe, and a shadow, with the tip of the probe, and the tip of the shadow being spaced by a distance related to the separation of the sample and the tip of the probe. Therefore, some parallax information was given.